The intention here is to explore what level of supply/demand matching between a renewable energy supply and electric vehicle charging could be achievable with different electric vehicle charging profiles. A higher level of supply/demand matching reduces the amount of backup or energy storage required to meet demand.

The first example starts by integrating electric vehicles with a simple night time charge profile. The second example then explores more constant charge profile throughout the day – this constant charge profile could be the result of a large number of electric cars all charging at different times, some at work, others at home over night. The third example explores a basic smart charging approach where the charge rate can aligned with the availability of renewable energy. There are many people who are already choosing their charge times to align with domestic solar pv output and there is much discussion about the opportunity that this may provide for matching supply and demand.

Night time charging

The first model investigates night time charging between 1am and 8am (7 hours). The charge profile of an aggregation of electric vehicles is much more likely to be smoother than this with a distribution over the day especially as the number of work based charging facilities and rapid chargers increase but for interest we will consider this case.

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

0.8699

0.5864

0.9931

1.1783

2.9825

Percentage of demandsupplied directly

27%

28%

29%

28%

3%

Flat charging profile

The results of running the model for a flat demand profile is the same as the earlier example where we considered the degree of matching between supply and a flat demand. Substantial improvements in matching compared to the night time charging profile is gained by just managing to distribute the charging requirements evenly across a day. This is unlikely to be possible on a single household basis but perhaps possible when the aggregate demand of many hundred cars are taken into account with day time charging at work encouraged.

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

0.8699

0.5864

0.9931

1.1783

2.9825

Percentage of demandsupplied directly

65%

76%

74%

57%

40%

Percentage of time demand ismore or the same as the supply

40%

46%

45%

38%

32%

Smart charging (variable rate charger that matches available supply)

Smart charging could allow electric vehicles that are left connected to the grid to charge when renewable electricity is available. The simple smart charging algorithm in this example starts by using available supply to charge the car's battery directly. A minimum SOC level required to cover the days journeys is maintained with a top-up charge if needed. The battery SOC is kept between 10% and 80% to help ensure long life is maintained.

The charge rate is based on a forecast of available supply over the next 24 hours, if the available supply is more than the forecast demand then the charge rate can be reduced. If there is twice as much supply forecast than demand then the rate of charge could by dropped to half the available supply in order to distribute the charge better across the 24h.

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

0.8699

0.5864

0.9931

1.1783

2.9825

Percentage of demandsupplied directly

78.7%

85.3%

80.3%

84.1%

72.2%

Percentage of time demand ismore or the same as the supply

69.5%

74.5%

67.5%

76.1%

69.2%

The results show substantial improvements again for the addition of smart charging, with smart charging + solar showing the largest gain. It is notable that the model suggests that a 2.98 kW solar PV array (a fairly typical amount for a home solar install) could provide 72.2% of almost 10000 miles a year of driving directly.

The feasibility of implementing this kind of variable rate smart charging on a household level with onsite solar pv needs a bit more investigation. The domestic charger on the nissan leaf can vary its charge rate in between 7A and 32A for the 6kW model or 7A and 13A for the 3kW model. Several people have already build open hardware variable rate electric car chargers making use of the fact that its possible to send a low voltage signal to an electric car to request a charge rate. Here are a couple of links to electric car charging related discussions and resources:

There are many aspects to consider and understand better when building a smart charger for electric vehicles for example it is suggested that to prolong the health of the battery its better to charge at a higher rate and up to the moment of starting your journey (rather than charge and let the battery sit at a high SOC). A more in depth understanding of the consequences of implementing this kind of charging and the balance point between battery life impacts and improved grid stability benefits or improved household economics would need to be understood.

Another possibility is that an aggregate demand profile for charging hundreds of electric cars could generally fit a renewable energy availability profile through a mixture of a larger portion of fixed rate charging cars charging at times of high availability than at other times.

In the next and final example in this zero carbon energy modelling series, we will combine the demand models for traditional electricity demand, electric heating and electric vehicles into one model and explore the implication of different electric vehicle demand profiles when interacting with multiple generation sources and multiple demand types.

With my Refurbished Samsung Galexy Tab 3 energy display up and running I've made a few changes to the solarPV application specific dashboard to make it work better. Thanks to Steve for many of the suggestions, see forum post.

The view now automatically updates as a rolling window, in any of the modes, 3 hour, 6 hour, day etc

The balance Import/Export is now the same size as the solar & house consumption.

Night time noise on the solar pv channel less than 10W is zeroed.

The in-window stats are now easier to see at a distance with larger font.

The view buttons are easier to click on a touch screen.

Its possible to make up the consumption or solar generation feed from multiple feeds on the fly by entering comma separated feed id's in the configuration interface.

We've been discussing for a little while using re-purposed tablets for energy display's rather than trying to develop our own pre-assembled version of the EmonGLCD (we're planning on keeping the through-hole version though).

I got myself a refurbished Samsung Galaxy tab 3 last week for £50 off ebay and the Koala Tablet Wall Mount Dock by Dockem.

It's fantastic when we get top hear about our energy monitors being used for applications we have never have thought of. Here is one such example:

SolarMill Writes:

We've just published our first open source project! It's called OpenEMC, and it's a code modification for the emonTH sensor by the OpenEnergyMonitor project. OpenEMC translates temperature and humidity readings into an easy to understand equilibrium moisture content (EMC) value, useful for woodworkers and operators of solar kilns.

The 5th example in the online zero carbon energy modelling tool is where it starts to get more interesting: modelling the supply/demand matching between a variable renewable supply and space heating delivered by heatpumps. The electrification of heating with heatpumps so that heating can be supplied with renewable electricity is one of the key solutions used in ZeroCarbonBritain and Sustainable energy without the hot air.

The ZeroCarbonBritain dataset includes weighted daily temperatures for 10 years. This dataset can be combined with the solar dataset and a basic household energy model to create a seasonal heating demand model.

Solar gains are an important aspect of space heating. Using MyHomeEnergyPlanner (The open source retrofit modelling tool we are developing with Carbon Coop) to model a concept low energy house with fabric energy efficiency of 120W/K and a total of 16m2 of window area on a 80m2 (floor area) house with external surface area 206.4m2 the maximum potential annual solar gains where calculated to be 4429 kWh.

MyHomeEnergyPlanner: http://github.com/emoncms/MyHomeEnergyPlanner

Running a low electricity demand of 2200 kWh a year and taking into account the solar gains. The space heating demand is only 4,247 kWh/year (compared to 13500 kWh/year for a typical house today a 68% space heating energy saving). If that remaining heat demand is supplied by a heatpump the electrical input should be 1,416 kWh/year.

Building an hourly space heating model:

In order to calculate the space heating demand the model first calculates the total heating demand before solar gains and internal gains are taken into account. The space heating demand assumes constant internal temperature target of 18.5C rather than a morning and evening heating period. A further example with a higher internal temperature and variable profile would worth exploring for comparison. (21.0C being the passivhaus internal temperature target and there is an interesting discussion about the role of heating profiles, heatpump performance and demand spikes)

The model then subtracts the internal gains (heat given off by appliances/cooking/lights etc) and the heat provided by solar gains through the windows, we use the solar pv capacity factor dataset here to provide our solar irradiance dataset. The amount of solar gain was scaled to match the amount of solar gain calculated in the SAP MyHomeEnergyPlanner tool based on the window orientations and areas – the total available solar gain energy is equivalent of 5.0 kW of solar pv.

The model also keeps track of unused solar and internal gains when the internal temperature is already at the target temperature. The assumption at this point is that the excess heat is dumped outside perhaps through increased ventilation.

The space heating demand after internal and solar gains are taken into account is then supplied with a heatpump with the simplifying assumption that the COP is constant and the heatpump fully responsive. A more complex model taking into account a degree of thermal mass in the building and the dynamics of the heatpump cross checked with real data would be useful here to check the assumptions taken in order to create an initial simplified model.

Running the same fabric energy efficiency, max available solar gains and internal gains through this hourly model gives a space heating demand that is 14% higher than the space heating demand calculated with the SAP based MyHomeEnergyPlanner. The difference may be due to the differences in the way solar gains and internal gains utilisation is calculated in a monthly vs hourly model, further investigation is needed to fully understand the reason for this difference.

Model heating demand results:

Total heat demand

8445 kWh/y

- Total utilized internal gains:

2044 kWh/y of 2201 kWh/y

- Total utilized solar gains:

1566 kWh/y of 4132 kWh/y

= Total space heating demand:

4835 kWh/y

Total heatpump electricity demand:

1611 kWh/y

Running the model for each renewable generation type to investigate the degree of direct supply demand matching we get the following results:

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

0.568kW

0.383kW

0.651kW

0.776kW

1.95kW

Percentage of demandsupplied directly

51%

57%

61%

43%

10%

Percentage of time demand ismore or the same as the supply

59%

59%

58%

54%

41%

Onshore, Offshore and Wave give quite similar levels of matching. Solar PV supplies the least demand because most of the solar electricity is generated in the summer and most of the heating demand is of course in the winter but also importantly when the sun is shining the heat demand is less due to direct solar gains, the dataset we are using for solar pv generation and solar gains is the same dataset.

The online example also explores the effect of adding a very basic thermal store in order to increase the level of supply matching.

The source code and datasets for the heating demand model and full supply/heating demand matching simulation is all open source available in both javascript and python.

We have just taken delivery of a batch of custom made Optical Utility Meter LED Pulse Sensor units. We're very excited about these new sensors, they will enable the emonPi and emonTx to interface directly with Utility Meters measuring exactly the amount of energy being measured by the utility meter.

The Optical Pulse Sensor works by sensing the LED pulse output from utility meters. Each pulse corresponds to a certain amount of energy passing through the meter. The amount of energy each pulse corresponds to depends on the meter. By counting these pulses the meters KWh value can be calculated.

Unlike clip-on CT based monitoring pulse counting is measuring exactly what the utility meter is measuring i.e. what you get billed for. The pulse counting cannot provide an instantaneous power reading like CT based can. Where possible we recommend using pulse counting in conjunction with CT monitoring. The emonPi and emonTx can simultaneously perform pulse counting and CT based monitoring.

In the future we plan to look at how the pulse counting energy value can be used to calibrate the CT based power calculations.

If you have backed us on Kickstarter or have purchased an emonPi from us as a thank you for your support we would like to offer you 20% off the Optical Pulse Sensor, use code PYN5031978E9T at checkout (valid until 1st July 2015).

We hear a lot that a renewable energy system benefits from having a mix of generating technologies. Combining wind and solar for example is said to provide a higher supply/demand matching than relying on one technology alone. When the wind isn’t blowing it may be sunny or vice versa.How do we work out the best mix of different renewable generating technologies. When is it cheaper to add more wind than to add more solar, what is the balance point for a particular demand profile?

This example explore's the balance point for onshore wind + solar, both having large resource availabilities associated with them. The mix will be balanced based on energy cost. It would also be good to explore the balance based on embodied energy. As with all modelling based on costs the outcome will change as costs change, the important thing here is to understand the method so that we can explore for a given set of costs what the optimum mix might be.In the recent contracts for a difference auction in the UK for renewable generation many of the onshore wind farms received a strike price of £82.50 per MWh. Two offshore wind projects received £115 per MWh and three solar farms received £79.23 per MWh. Source: Contracts for Difference Auction Results

In this example we will use these cost figures, the ZCB capacity dataset for onshore wind and solar and a simple flat demand profile.

If we look at the results from example 2 investigating annual matching for wind and solar and add in the cost information:

1.164kW of onshore wind delivers 3300 kWh/y at £272/y and a supply/demand matching of 65.88%.

3.99kW of solar delivers 3300 kWh/y at £261/y and a supply/demand matching of 40.61%.

One way to investigate the best mix is to fix the total annual energy cost and change the installed capacities of both solar and wind to achieve the greatest level of matching for a given energy cost.

So lets take an annual energy cost of £272 and work out for this cost what is the maximum level of matching we can obtain from a wind + solar mix.

At an energy cost of £272/year a flat demand and the ZCB dataset we can see a clear benefit from combining solar and wind in the energy mix, increasing solar pv capacity appears to make sense up to 105.8% of installed wind capacity after which the matching starts to drop again for the given energy cost.

Its important to note however that wind still provides the majority of the electricity at 2543 kWh of the 3300 kWh generated annually (76%). This is because of wind's higher capacity factor in comparison with solar.

How does the mix change if we decide to oversupply and pay a higher cost for the electricity. If we fix our annual cost to say £320

Cost

Wind capacity

Solar capacity

Matching

£320.06

1.369

0

69.88 %

£320.07

1.229

0.5

73.98 %

£320.08

1.144

0.8

75.20 %

£320.08

1.089

1.0

75.44 %

£320.08

1.075

1.05

75.45 %

£320.08

1.061

1.1

75.44 %

£320.08

1.047

1.15

75.41 %

£320.08

1.033

1.2

75.37 %

The maximum matching we obtained in this case happened where solar capacity was 97.7% of wind capacity.

It appears that in these model runs, the optimal mix between solar and wind is to install an equal capacity of both, its interesting that this happens to be the case and that its not say half the wind capacity. The model results confirm the often discussed complementarity between solar and wind supply and that the benefit of their combination increases supply demand matching by around 5% points for no additional cost and is a similar scale of supply/demand matching improvement seen by increasing the oversupply of wind to 120% of demand but without the additional cost.

The ZeroCarbonBritain dataset includes 10 years of hourly traditional electricity demand data for the UK. The previous example compared renewable supply data with a flat demand profile, this example explores the effect of the variable traditional electricity demand profile with its day time peaks and night time low on supply/demand matching for the different renewable energy generators.

The screenshot below gives a flavour for what the traditional electricity demand profile looks like in blue, the black line is the supply from onshore wind, using the tool you can compare traditional electricity demand to: onshore wind, offshore wind, tidal, wave and solar power.

These are the results for the amount of demand supplied directly for each generation type, matching annual supply totals with demand totals:

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

1.17kW

0.79kW

1.33kW

1.58kW

3.98kW

Percentage of demandsupplied directly

66.5%

76.7%

75.2%

57.0%

42.1%

Percentage of time demand ismore or the same as the supply

40.7%

46.5%

44.7%

38.7%

31.1%

Interestingly they only change marginally. Solar PV makes a gain 2% on the demand supplied directly which reflects higher demand in the day vs night time and we see a couple of other 1% changes but the differences are quite marginal and smaller than the difference between each renewable energy type so we don’t really see any change of order.

Increasing the degree of supply/demand matching between a variable renewable supply and traditional electricity demand by over supply

The are multiple ways of increasing the level of supply/demand matching or reducing the unmet demand. Over-supply is one way we can do this and is one of the measures used in the ZeroCarbonBritain scenario. In the previous examples we sized the installed capacity of the renewable electricity generating technologies to produce over the 10 year model period the exact same amount of electricity as was used in the 10 year period.We can re-run the same model but with installed capacity amounts set to 110%, 120% or 130% of demand

Oversupply: 110%

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

1.28kW

0.86kW

1.46kW

1.74kW

4.39kW

Percentage of demandsupplied directly

68.9%

79.6%

77.9

58.9%

43.0%

Percentage of time demand ismore or the same as the supply

44.1%

51.8%

50.0%

41.7%

32.4%

Oversupply: 120%

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

1.28kW

0.94kW

1.60kW

1.89kW

4.79kW

Percentage of demandsupplied directly

71.1%

81.9%

80.3%

60.4%

43.8%

Percentage of time demand ismore or the same as the supply

47.2%

56.5%

54.3%

44.3%

33.5%

Oversupply: 130%

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

1.51kW

1.02kW

1.73kW

2.05kW

5.19kW

Percentage of demandsupplied directly

72.9%

83.9%

82.2%

61.7%

44.5%

Percentage of time demand ismore or the same as the supply

50.1%

60.7%

58.1%

46.7%

34.4%

For every 10% of demand increase in supply we see 1-3% improvements in the percentage of demand supplied directly and 1-5% improvements in the amount of time demand is more or the same as supply.

The next example looks at the question of complementarity between different renewable energy types and asks the question what might the optimum capacity mix point be between wind and solar for a given electricity price point.

The second example in the hourly energy modelling tool models the degree of supply/demand matching between a variable renewable supply consisting of a single renewable energy generation type and a flat electricity demand profile.

A flat demand may not of course be particularly realistic and the more complex examples later on address this, but I've used it here just to illustrate this particular simple example case.

Online tool:http://openenergymonitor.org/energymodel > navigate to 2. variable supply and flat demandThe demand is subtracted from the supply for every hour in the 10 year period and the total amount of unmet demand and excess generation is measured as well as the amount of time the supply was more than or equal to the demand.

The demand level is set to an annual average electricity demand of 3300 kWh which is the average UK household annual electricity consumption. The amount of installed capacity is set to match this demand on the 10 year basis of the dataset.

Running the model for each of each generation type, matching total 10 year supply to total 10 year demand of 3300 kWh x 10 we get the following results:

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

1.17kW

0.79kW

1.33kW

1.58kW

3.98kW

Percentage of demandsupplied directly

65.9%

76.4%

73.9%

57.7%

40.6%

Percentage of time demand ismore or the same as the supply

40.1%

46.2%

45.3%

38.6%

32.1%

We can see again here that offshore wind is the clear winner with the lowest installed capacity requirement and highest level of supply/demand matching. Perhaps an interesting result is how less predictable technologies such as wind and wave provide greater levels of matching than power from tidal which is very predictable.

Python example source codeAlongside the online javascript modelling tool there are a series of python versions of the examples which are simpler to follow as they dont include all the code to create the visual output, they just print out the main results at the end.

This first example in the hourly energy modelling tool model's the hourly output of a given installed capacity of wind, wave, tidal or solar. The model really isn’t doing much its just loading the capacity factors for every hour from the 10 year dataset and multiplying the capacity factor by the installed capacity.

The total electricity generated is calculated as the sum of the electricity generation in each hour and printed along with the capacity factor at the end.

The units are really not important the example could just as well be in MW's or GW's. kW's where chosen as the other model examples in the series are focused around building a hourly model that's relatable to an average households energy demand. The kW's of installed capacity could just relate to a small share of a much larger wind turbine, solar farm, wave or tidal power installation.

Running the model for each of these generation types with the same installed capacity the results are as follows:

Onshore wind

Offshore wind

Wave

Tidal

Solar

Installed capacity

1.0kW

1.0kW

1.0kW

1.0kW

1.0kW

Annual generation

2834 kWh

4204 kWh

2482 kWh

2092 kWh

826 kWh

Capacity factor

32%

47%

28%

23%

9%

In all of the examples above the installed capacity of the renewable generator was the same (1.0kW) but we can see straight away that there is a significant difference in the total electricity generated by each type. A unit of offshore wind generates just over 5 times as much energy as a unit of solar pv using the zerocarbonbritain dataset. By itself this is not enough information to evaluate the effectiveness of a technology, we would need to compare the costs per unit of installed capacity, embodied energy per unit of installed capacity, how well a particular solution matches demand, the land areas required, the availability of the resource to name just a few of the many factors that need to be weighed up but it does highlight one of the important factors.

Python example source codeAlongside the online javascript modelling tool there are a series of python versions of the examples which are simpler to follow as they dont include all the code to create the visual output, they just print out the main results at the end.

The following 19 lines of python code are all you need to load the ZeroCarbonBritain dataset and run through all 87,648 hours, calculating the power output for each hour and accumulating the total energy supplied over the 10 year model period: